The present invention relates to electrostatic chucks. Semiconductor wafers need to be securely held in place during processing. In some wafer processing machines, the wafer is held in place by an electrostatic chuck. An electrostatic chuck uses an electrostatic force to hold the wafer. The electrostatic force is produced by a voltage difference between the wafer and a conductive portion of the chuck across a dielectric portion of the chuck. Electrostatic chucks are typically made of a metal material with a thin dielectric coating. For example, an aluminum chuck with an aluminum-oxide coating could be used. Alternately, the dielectric layer could be formed of a polyamide or ceramic material.
FIG. 1A shows a prior art unipolar chuck 12. The chuck 12 includes a conductive portion 12a and a dielectric portion 12b. The wafer 14 is placed on the chuck 12. The metallic portion 12a is positively biased by DC power supply 11 and the plasma produced in the chamber 10 supplies electrons onto the wafer 14. The electrons in the wafer 14 migrate toward the positively biased metal portion 12a of the chuck 12 but cannot cross the dielectric layer 12b. The voltage difference across the dielectric layer 12d will produce an electrostatic force which will hold the wafer 14 to the chuck.
FIG. 1B shows a bipolar chuck 18. The bipolar chuck has two poles: pole 18a and pole 18b. These poles are separated by a dielectric layer 18c and the chuck is covered by the dielectric layer 18d. In the bipolar chuck, DC power supply 21 is used to make one of the poles, 18b, positively charged and the other pole 18a negatively charged. This will cause mobile electrons in the wafer 20 to migrate into regions 20c and 20b away from region 20a. This will cause the region 20a to become positively charged. In this manner, an electrostatic force is produced between the wafer 20 and the chuck 18.
Typically, both types of electrostatic chucks include air flow grooves for cooling the wafer, liquid cooling pipes for cooling the chuck, and screw holes for attachment of the chuck to the chamber. The manufacturer typically includes a number indicating the highest level of air flow that can be used with the chucks.
It is desired to have a method of testing wafer chucks.